Clustering effects of lone star ticks in nature: implications for control.

Introduction

To obtain a blood meal, a hard tick must attach to a vertebrate host for several days. Different tick species use hosts such as dogs, cats, deer, raccoons, opossums, horses, cattle, and human beings. Since engorged ticks are most likely to fall off wherever their hosts spend the most time, the presence of ticks in the environment is largely a function of host activity. Typical tick habitats include brushy, weedy, or forest environments. Ecotones, areas that lie between climax (mature) forests and open fields, are especially common habitats. The spatial arrangement of ticks may be uniform (underdispersed), random, or aggregated (over-dispersed)(1), depending on tick species and host.

Previous tick studies in Mississippi have indicated clustering of at least two hard-tick species. A study of the ecology of adult Ixodes scapularis in a privately owned wildlife management area demonstrated clustering patterns (2). Another study, 200 miles north of the first, revealed clumping of adult Amblyomma americana, or lone star ticks (LSTs)(3). It is important to determine whether tick clustering is real and predictable, and not just an artifact of sampling methods. For example, if a collector is dragging a cloth for ticks, the frequency with which the cloth is turned and examined for ticks may determine if several ticks appear to be dumped together [ILLUSTRATION FOR PHOTO 10 OMITTED]. If clustering is distinct and predictable, then control strategies can be directed toward finding and spraying only the areas of tick clustering.

Several tick-borne diseases occur in Mississippi (Rocky Mountain spotted fever [RMSF], Lyme disease, ehrlichiosis, and tick paralysis). Approximately 20 to 30 cases of RMSF and about 10 to 20 cases of Lyme disease are reported in the state annually. Ehrlichiosis and tick paralysis are reported only rarely. Although cases of tick-borne disease are few relative to other diseases, they are consistent sources of the morbidity and mortality that result from tick bites each year. RMSF is especially severe, with a fatality rate of approximately five percent (even with treatment). One goal of the entomology program at the Mississippi Department of Health is to explore ways to reduce tick-human exposure and ultimately to reduce the number of cases of tick-borne disease. With the broader objective of setting up large-scale tick control projects in the Mississippi state parks, several small initial studies are being undertaken to evaluate the ecology of ticks in Mississippi and determine ways to precision-target pesticides. This is the first report of the initial studies; it is designed to document the distribution of adult and nymphal LSTs in a recreational area and to determine whether distribution is related to soil moisture and amount of overhead shade.

Materials and Methods

In late winter, 1994, two lanes 620 meters long by 2 meters wide were bush-hogged through the woods at the Copiah County Game Management Area in central Mississippi. The narrow lanes were bush-hogged to facilitate careful sampling and exact numbering of each lane into 10-meter sections. The two lanes were located 1.6 kilometers apart. Each lane crossed several types of habitat (open field; creek bottoms; and forests of oak, hickory, and pine). From April 1, 1994, to October 31, 1994, each lane was sampled for ticks on a weekly basis: a 1-square-meter piece of white flannel cloth was dragged up one side of the lane all the way to the end and back down the other side. The doth was examined for ticks at intervals of 10 meters. Spots were marked on a map wherever ticks were captured. Except for voucher specimens, all ticks were returned alive to the plots.

TABLE 1
Clustering of LSTs at Study Sites

Site                     Tick Stage             % of Area Where
                                             Majority Ticks Found

1                           Adult                     17.7
1                           Nymph                      9.7
2                           Adult                     14.5
2                           Nymph                     25.8
TABLE 2
Percent Shade Where Majority of LSTs Were Collected

Site       Tick Stage       Number of LSTs Collected       % Shade

1             Adult                21/31 (68%)                71
1             Nymph                24/33 (73%)                65
2             Adult                31/44 (70%)                45
2             Nymph                81/113 (72%)               71

Percent shade for each 10-meter section of lane was visually estimated three times during the season (May 15, July 15, and September 15) and averaged to yield a shade value for each section. Percent soil moisture was determined (using an OSKR soil moisture meter from Forestry Suppliers of Jackson, Mississippi) on the same three dates and averaged to yield a moisture value for each 10-meter section.

Results and Discussion

Overall, 221 LSTs were collected (64 from Site 1 and 157 from Site 2). Breaking down by life stage, 31 adults and 33 nymphs were collected from Site 1; 44 adults and 113 nymphs were collected from Site 2. The seasonal breakdown was as follows: the peak for adult ticks at Site i was June 15 and the peak for Site 2 was June 8. More nymphs were collected on May 31 and on June 8 than on any other date at Site 1, and at Site 2, more nymphs were collected on June 8 than on any other date.

Lone star ticks were clustered in the lanes, most obviously at Site 2 - especially the nymphs [ILLUSTRATION FOR FIGURE 1 OMITTED]. In fact, when the size of the area where most ticks were found is calculated and divided by the total area (times 100), the percent of area where ticks were found proves quite small (Table 1). This was most striking for nymphs in Site 1, where 73 percent of the nymphs were collected in 9.7 percent of the lane. Overall, approximately 70 percent of LSTs were found in approximately 10 to 20 percent of the geographic area.

The amount of shade in the lanes ranged from 0 to 90 percent at both sites. Ticks were collected mostly from shaded areas. Only three ticks out of 221 were collected in 10 percent or less shade. This may be due to the inability of LSTs to survive hot, dry environments. One study in Arkansas demonstrated that LST eggs reared in an environment of less than 75 percent humidity would not hatch (4). The average amount of shade for all areas where adult LSTs were caught was 63 percent at Site I and 43 percent at Site 2. The average amount of shade for all areas where nymphs were collected was 61 percent at Site 1 and 46 percent at Site 2. If one further breaks down the collection data into only the areas where most LSTs were collected, the percentages are higher (Table 2). Soil moisture in the lanes ranged from 20 to 66 percent at Site 1 and from 27 to 88 percent at Site 2. Overall, Site 2 was a wetter site. Interestingly, Site 2 produced 30 percent more ticks than Site 1. Again, this may relate to the moisture needs of LSTs. Ticks were collected where soil moisture was greater than 23 percent, with most collected where it was 37 percent or higher (Table 3).

Implications for Control

Although limited (field work may yield variable results because of weather patterns), this study suggests that LST clustering in nature is real and somewhat predictable. Clearly definable spots in the study sites consistently had ticks, whereas others did not. For example, almost no ticks were collected in open areas with full sunlight.

TABLE 3

Percent Soil Moisture Where Majority of LSTs Were Collected

Site      Tick Stage     Number of LSTs Collected%    Soil Moisture

1           Adult              21/31 (68%)                  38
1           Nymph              24/33 (73%)                  37
2           Adult              31/44 (70%)                  52
2           Nymph              81/113 (72%)                 53

Approximately 70 percent of ticks were collected in 10 to 20 percent of the geographic area. This clustering phenomenon can be exploited for control purposes. If that 10 to 20 percent of area (say, inside a state park, for example) were to be identified by surveillance with a drag cloth in the early spring and sprayed from a backpack sprayer with an appropriate pesticide, then theoretically tick-human contact would be reduced with only limited use of pesticides. Further, if spots of clustering can be accurately predicted based upon habitat or vegetation type, then those spots can be found and treated even earlier in the spring, possibly without drag-doth surveillance. More research on the ecology of hard ticks will help validate or invalidate these hypotheses.

REFERENCES

1. Korch, G.W. Jr. (1994), "Geographic Dissemination of Tick-Borne Zoonoses," Ecological Dynamics of Tick-Borne Zoonoses, ed. D.E. Sonenshine and T.N. Mather, Oxford University Press: New York, NY.

2. Goddard, J. (1992), "Ecological Studies of Adult Ixodes scapularis in Central Mississippi: Questing Activity in Relation to Time of Year, Vegetation Type, and Meteorologic Conditions," J. Med. Entomol., 29(3):501-506.

3. Jackson, L.K., D.M. Gaydon, and J. Goddard (1996), "Seasonal Activity and Relative Abundance of Amblyomma americanum in Mississippi," J. Med. Entomol., 33(1):128-131.

4. Lancaster, J.L. (1957), "Control of the Lone Star Tick," U. Ark. Agric. Experiment Station Report Service 67:1-16.

Corresponding Author. Jerome Goddard, Ph.D., Bureau of Environmental Health, Mississippi Department of Health, P.O. Box 1700, Jackson, MS 39215.